Acoustic probes can include a first transducer that generates oscillatory flow, such as sound waves, within a volume and a second transducer that measures an acoustic pressure within the volume. Among other things, the acoustic probes can be used to measure acoustic pressures within the human ear canal. Recent movement in the field of hearing research and testing has included work in calibrating acoustic stimuli in units of the component of the sound pressure that travels toward an eardrum, or forward pressure, thus minimizing the impact of standing waves created by interaction between the forward pressure and the component reflected from the eardrum. Such calibration can be superior in controlling stimulus levels to an ear than some other known methods.
In order to calibrate an acoustic probe, the Thévenin source pressure and impedance of the acoustic probe can be determined using a series of cylindrical cavities or waveguides of different lengths. For example, a series of rigid tubes of different, or stepped, lengths may be used. The brass tubes can be fixed to a rigid base plate and the acoustic probe manually inserted into each of the tubes in succession. When inserted into a tube, the first transducer of the acoustic probe emits acoustic stimuli (e.g., a series of pure tones covering a frequency range of interest, MLS sequences, pseudorandom noise, chirps, clicks, and the like) and the second transducer measures the pressure response inside the tube to the acoustic stimuli.
The pressure responses are measured for several or all of the tubes and can be used to solve for the Thévenin source parameters, its source pressure and impedance. The Thévenin source parameters can then be used to calibrate measurements obtained by the acoustic probe in human ear canals. The Thévenin calibration of acoustic probes can be used to improve the accuracy of a variety of hearing measurements in research laboratories, hearing clinics, industrial hearing conservation programs, infant screening programs, school children screening programs, and the like.
However, the known manually intensive calibration procedures of acoustic probes can require significant human interaction and work with both the acoustic probes and the waveguides used to measure the pressure responses. As the amount of manual work involved in calibrating an acoustic probe increases, the amount of time required to calibrate the probe and the possibility of error can increase.